Stripping of sulfur dioxide from gas streams by use of n-alkyl lactams

ABSTRACT

Sulfur dioxide is stripped from a gas stream by N-alkyl lactam, particularly an N-lower alkyl pyrrolidone, e.g., N-methyl pyrrolidone, sufficiently to permit the non-polluting discharge of the gas stream to the atmosphere. Gas streams containing from about 0.05 percent to about 10 percent sulfur dioxide by volume can thus be passed at a gas mass flow velocity of 200-3,000 lbs/hr/ft2 through an N-alkyl lactam-containing contacting zone of 5-85 ft in length, to reduce the sulfur dioxide content thereof to less than about 250 ppm, advantageously to less than 100 ppm, or even 50 ppm, prior to discharge to the atmosphere. Gas streams having SO2 contents of from about 0.01 percent to about 50 percent by volume can thus be treated to achieve at least about 90 percent SO2 removal by volume, desirably at least about 95 percent, and advantageously at least about 98 percent removal. Gas temperatures of about 0*-75*C, preferably about 35*65*C, are employed. Lactam entrained in the gas stream discharged from the contacting zone is conveniently recovered in a water scrubbing zone. Lactam from the contacting zone and lactam in water removed from the scrubbing zone can conveniently be recovered in a distillation zone by heating to 100*-200*C. The lactam recovered can be recycled to the contacting zone, and the SO2 stripped from said lactam can be conveniently recovered for non-polluting disposal or use.

United States Patent [191 Bellisio et a].

[541 STRIPPING F SULFUR DIOXIDE FROM GAS STREAMS BY USE OF N ALKYLLACTAMS [75] Inventors: Arthur A. Bellisio, Huntington Station, N.Y.;Hippocrates G. Psyras, Berkeley Heights; Marvin M. Fein, Westfield, bothof NJ.

[73] Assignee: GAF Corporation, New York, N.Y.

Primary Examiner-Charles N. Hart Attorney-Walter C. Kehm et al.

WATE R [111 3,733,779 [451 May 22, 1973 [57] ABSTRACT Sulfur dioxide isstripped from'a gas stream by N-alkyl lactam, particularly an N-loweralkyl pyrrolidone, e.g., N-methyl pyrrolidone, sufficiently to permitthe nonpolluting discharge of the gas stream to the atmosphere. Gasstreams containing from about 0.05 percent to about 10 percent sulfurdioxide by volume can thus be passed at a gas mass flow velocity ofZOO-3,000 lbs/hr/ft' through an N-alkyi lactam-containing contactingzone of 5-85 ft in length, to reduce the sulfur dioxide content thereofto less than about 250 ppm, advantageously to less than 100 ppm, or evenppm, prior to discharge to the atmosphere. Gas streams having SO,contents of from about 0.01 percent to about 50 percent by volume canthus be treated to achieve at least about percent SO, removal by volume,desirably at least about percent, and advantageously at least about 98percent removal. Gas temperatures of about 0-75C, preferably about35-65C,' are employed. Lactam entrained in the gas stream dischargedfrom the contacting zone is conveniently recovered in a water scrubbingzone. Lactam from the contacting zone and lactam in water removed fromthe scrubbing zone can conveniently be recovered in a distillation zoneby heating to l00-200C. The lactam recovered can be recycled to thecontacting zone, and the S0, stripped from said lactam can beconveniently recovered for non-polluting disposal or use.

36 Claims, 1 Drawing Figure RECYCLE VAPOR n. nw 49 SCRUBBER 22- ZONE.

STILL X4] WATER STRIPPING OF SULFUR DIOXIDE FROM GAS STREAMS BY USE OFN-ALKYL LACTAMS CROSS-REFERENCE TO RELATED APPLICATION The subjectapplication is a continuation-in-part of applicants co-pendingapplication, Ser. No. 872,755, filed Oct. 30, 1969, entitled REMOVAL OFSULFUR DIOXIDE USING NeALKYL LACTAM.

BACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention relates to the stripping of sulfur dioxide from a wet or drygas stream. More particularly, it relates to the stripping of sulfurdioxide impurities sufficiently to permit the non-polluting discharge ofthe treated gas stream to the atmosphere.

2. Description of the Prior Art While the removal of sulfur dioxide fromstack gases and similar gases containing this gaseous component hasalways been a problem, the awareness of this prob lem has become moreacute in recent years with the increasing concern about air pollution.Thus, with the increased utilization of high sulfur-containing fuels,there has been an ever increasing pollution of the atmosphere with thecombustion products of sulfur, namely sulfur dioxide. Accordingly, thereis a substantial desire in the art for the production of a novel systemby which the sulfur dioxide impurities can be removed from stack gasesand other SO -containing gases, such as the gases produced from theburning or combustion of high sulfur-containing fuels in power plants,as well as from paper mill and smelter gases, and from hydrogensulfide-based sulfur plants, sulfuric acid plants, and the like.

In addition to such air pollution considerations, great interest alsoexists with reference to recovering, for subsequent use, sulfur dioxidefrom the combustion gases obtained when sulfur or high fuels are burned.The sulfur dioxide thus recovered would be utilized as by, for example,further oxidation to sulfur trioxide for the production. of sulfuricacid. It is likewise possible, particularly when the sulfur dioxide isrecovered from the stack gases produced from the combustion of highsulfur fuels, to react the recovered sulfur dioxide with hydrogensulfide to produce sulfur. Accordingly, in addition to the desire tokeep the air free from pollution and free from unwanted sulfur dioxidecontamination, the commercial utilization of the recovered sulfurdioxide to produce other valuable products is of considerableimportance.

The ever increasing awareness and concern relating to the environmentalaspects of atmospheric emissions, and the desire to recover and utilizevaluable byproduct materials, relate not only to sulfur dioxide, but toall sulfur-containing industrial waste streams being discharged to theatmosphere. As a result, numerous processing techniques have beenproposed for the removal of sulfur dioxide, hydrogen sulfide and othersulfur-containing gases from a variety of industrial gas streams. Whileall such proposals are of interest and many may offer promise inreducing atmospheric pollution, the recognition of the environmentalconsequences of all aspects of industrial waste disposal suggest andeven compel that further advances be made with respect to industrial gasdischarge operations. Thus, regulations controlling the discharge ofsulfur contaminants to the atmosphere have been made more 2,restrictive, with further tightening of such regulations awaiting onlythe development of more effective pollution control techniques renderingsuch more restrictive standards commercially feasible for incorporationin industrial process operations.

With respect to the removal of hydrogen sulfide from natural gas,refinery waste gases and other industrial gas streams, the use ofN-alkylated lactarns have been proposed. Thus, Thormann et 2.1., U. S.Pat. No. 3,120,993, disclose the absorption of hydrogen sulfide andorganic sulfur compounds from a gas stream by counter-current contactwith a stream of N-alkylated lactam absorbent as disclosed in Column 4,line 33 et seq. The Kohrt US. Pat. No. 3,324,627, also discloses the useof methyl pyrrolidone as a solvent for hydrogen sulfide.

In the Fuchs U. S. Pat., No. 3,103,411, such hydrogen sulfide removalfrom gas streams by means of N- alkyl pyrrolidone and like materials isfurther disclosed, with the hydrogen sulfide thus removed by anoxidation with oxygen, S0 or gases containing the same to produceelementary sulfur. For this purpose, the gas stream to be treated iscontacted with liquid N-alkyl pyrrolidone at temperatures of -20 to+l50C so as to dissolve hydrogen sulfide in the liquid pyrrolidone.Since S0 rather than molecular oxygen can be employed for reaction withthe dissolved H 8 to produce the desired elemental sulfur in the N-alkylpyrrolidone, Fuchs teaches that H s-containing gases that also containsulfur dioxide, such as Claus contact furnace waste gas, are especiallyuseful. Fuchs discloses various examples in which S0 is passed intoN-alkyl pyrrolidone containing the H 8 stripped from a waste gas stream,the S0 and H 8 reacting to produce the elemental sulfur product. InExample 11, Fuchs indicates that S0 can be dissolved in a separate bodyof N-methyl pyrrolidone, as from a roaster gas containing 8.1% S0 byvolume, with this body of lactam being united with a separate body oflactam containing the H 5 dissolved therein, the H 8 being oxidized tosulfur upon mixing of the two bodies of lactam material.

Fuchs discloses that the H 8 content of the natural gas stream treatedin this example is reduced from 12.6 percent by volume to less than 0.05percent by volume, i.e. 500 ppm. In other examples, Fuchs indicates thatthe waste gas upon treatment was free of H 8.

Techniques have also been proposed for general applicability to theremoval of acid gases from gas streams by means of a variety of solventcombinations and particular operating procedures. Thus, British Pat. No.1,196,610 discloses the purification of gas streams con taining CO andother gases, H 8, etc., employing, as disclosed on page 2, line 34 etseq., secondary monohydric alkanolamine solvent and a second organicsolvent, other than an alkanolamine, having a specified solubility forCO a boiling point of at least C and a specified non-hydroxyl functionalgroup. Preferred second solvents specified include cyclic and acyclicethers, glycol ethers, dioxolanes, dioxanes, furans, trioxanes,'oxazoles, etc., sulfones and sulfoxides, substituted andunsubstituted pyrroles such as 2-pyrrolidone and N-methyl pyrrolidone,with Sulfolane and dioxolane, e.g., 2,2-dimethy1-4-hydroxymethyl-l ,3-dioxolane being particularly preferred. Table II shows that thecombination solvent, in its particularly preferred embodiments,permitted a given CO absorption to be accomplished in a smallergas-liquid contact time than in the comparison solvent combinationsemployed.

In British Pat. No. 1,234,862, in the name of Claude Dezael, anapparatus is disclosed for separating gas mixtures by liquid solvents inwhich at least two extraction zones and at least two decanting zones areprovided for greater efficiency and higher flow rate operation. Dezaeldiscloses absorbing a wide variety of gases from gaseous mixtures bypassage through the disclosed apparatus, such gases including CO H 8, SS0 C1 HCl, N0 NH, and C H- Solvents such as alkanolamines, amino acids,water, aqueous solutions of amines, ammonia or mineral salts,dimethylsulphoxide, N-methyl pyrrolidone, glycol, polyglycols, ethers,esters, Sulfolane and other solvents of industrial use are disclosed asbeing suitable at page 2, line 116 page 3, line 15. As in the Fuchspatent referred to above, the reaction of absorbed H 8 and S0 to producesulfur is disclosed. In Example 1, ethanolamine dissolved indimethylsulphoxide is used to remove CO from hydrogen. The dischargedhydrogenv gas stream was said to contain no more than a trace of CO thetrace being indicated as less than 0.1 percent by volume, i.e. 1,000ppm. ln Example 2, a solvent combination of 50 parts by volume ofethanolarnine and 50 parts by volume of N-methyl pyrrolidone was used toextract CO from nitrogen, the effluent gases being said to consistessentially of nitrogen.

Numerous other references attest to the commendable effort being made inmany areas of industrial activity to develop air pollution controls andby-product recovery techniques relating to various sulfur contaminantsof industrial gas streams. The stripping of sulfur dioxide from gasstreams, such as from combustion gas streams, however, remains a majorproblem of genuine environmental concern. This concern and the desirefor more restrictive air pollution control regulations are expressed inChemical Engineering, June 14, 1971, pages 58, 60 and 62 in an articleentitled Sulfurrecovery Processes Compete for Leading Role. As indicatedtherein, regulations adopted or under consideration in major UnitedStates cities for sulfur dioxide stack specifications range from 100 ppmtoSOO ppm of S0,. It is also indicated that commercial feasibility hasnot been proved for any of the variety of proposals being considered,although the 1970 Clear Air Act mandates that the best availabletechnology" be utilized by 1975 and that the standards adopted be met,rather than waiting for ideal processes to become available. It is alsostated in this article that the ability to regenerate the reagentemployed, whatever it may be, for recycle and reuse appears to be anecessary requirement for a successful pollution control process.

Chemical Week, of Dec. 8, 1971, pages 33-34, provides a furtherindication of the industrial significance of proposals for morerestrictive emission standards. This article also points out that 2,000ppm has been the standard adopted by most states up until the presenttime, the more restrictive standards under consideration beingconsiderably lower than the present ones and necessitating a highlyeffective technique for reducing emissions without an unduly adverseeffect on the economic feasibility of the processes into which it isincorporated.

There is, therefore, a highly critical andv urgent need for newtechniques for controlling sulfur dioxide emissions to the atmosphere inindustrial operations. New

techniques for this important purpose must, however, accomplish thehighly desirable control of such emissions in an economically acceptablemanner, particularly providing for the ready regeneration and recycle ofthe materials employed so as not to obviate the economic justificationfor continuing the basic operation in a non-polluting manner inaccordance with the more restrictive emission control requirementspresently contemplated and desired. A further highly desirable featureof such a pollution control technique would be the ability to utilizethe sulfur values recovered from stack gases for use in practical,non-polluting applications so as to further minimize the economicconsequences resulting from a successful effort to reduce sulfur dioxideemissions to the levels presently contemplated for environmentalprotection of the atmosphere.

It is an object of the present invention, therefore, to provide animproved process for the selective removal of sulfur dioxide frommixtures of gases containing the same.

It is another object of the invention to provide an improved process forsaid selective removal of sulfur dioxide frorn a wet or dry gaseousstream.

It is another object of the invention to provide a process for theeffective stripping of sulfur dioxide from gas streams sufficiently topermit the non-polluting discharge of said gas streams to theatmosphere.

It is another object of the invention to provide a process for thestripping of sulfur dioxide from gas streams to be discharged to theatmosphere to less than about 250 ppm, advantageously to less than about100 ppm.

It is a further object of the invention to provide a pro cess for thestripping of sulfur dioxide from gas streams by means of a liquidsolvent capable of being regenerated and recycled for further use.

It is a further object of the invention to provide a process for thestripping of sulfur dioxide from gas streams and for the recovery ofsaid sulfur dioxide in a form suitable for subsequent non-pollutingdisposal or use.

These and other objects are achieved by the invention as hereinafter setforth in detail, the novel features thereof being particularly pointedout in the appended claims.

SUMMARY OF THE INVENTION N-alkyl lactam, particularly N-lower alkylpyrrolidone, such as N-methyl pyrrolidone, is employed to strip sulfurdioxide from a gas stream sufficiently to permit the non-pollutingdischarge of the gas stream to the atmosphere. The remarkableeffectiveness of N- alkyl lactarn for this purpose permits gas streamsflowing at commercially feasible flow rates to be treated in acontacting zone of economically practical size to strip therefrom, at075C, preferably about 35C to about 65C, to permit a gas stream havingfrom about 0.5 percent to about 10% S0 by volume to be discharged to theatmosphere with an S0, content of less than about 250 ppm,advantageously less than 100 ppm. Gases having a wider range of sulfurcontent, e.g. 0.01 percent to about 50 percent by volume, can thus betreated by N-alkyl lactam under such commercially feasible conditions toachieve at least about percent by volume, desirably at least aboutpercent or even 98 percent or above, stripping of the S0 content of thegas stream being treated.

The N-alkyl lactam having SOgabsorbed therein can readily be regeneratedby heating to 100200C to BRIEF DESCRIPTION OF THE DRAWING The presentinvention is hereinafter set forth in further detail with reference tothe accompanying drawing that constitutes a flow diagram representing anembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION In the practice of the presentinvention, a gas stream containing sulfur dioxide impurities is passedthrough an absorption zone for contact with a liquid alkyl lactamsolvent. The lactam employed is highly selective for sulfur dioxide andhas a high sulfur dioxide absorbing ability over a specified temperaturerange. These desirable solubility characteristics of the lactam areaccompanied by additional features of great significance, permitting thelactam solvent to be regenerated for recycling to the absorption zonefor contact with additional quantities of sulfur dioxide containing gas.Thus, the high solubility of the lactam for sulfur dioxide decreasessharply at higher temperatures as hereinafter indicated. The lactam iscapable of withstanding such higher temperatures without degradation,and the low vapor pressure of the lactam allows ease of design forprevention of undesired loss of lactam during the regenerationoperation.

Gas streams having a wide range of sulfur dioxide impurity contents canbe treated, in accordance with the present invention, under commerciallysignificant conditions, to strip the sulfur dioxide impurities from sandstreams. Thus, gas streams containing from about 0.1 percent to about 50percent sulfur dioxide by volume can be contacted with a liquid N-alkyllactam solvent under the conditions hereinafter set forth to remove atleast about 90 percent by volume of the sulfur dioxide impuritiespresent in the untreated gas stream. The gas stream discharged from thecontacting or absorption zone with the sulfur dioxide impurities verysubstantially washed therefrom can subsequently be utilized or furthertreated in an appropriate manner. Where the sulfur dioxide impuritycontent of the treated gas stream is less than the applicable airpollution control regulations, the non-polluting discharge of thetreated gas stream to the atmosphere would be permissible. The treatedgas stream referred to above, it will be noted, include gas streamshaving s sulfur dioxide content of less than those of the existing andmany of the contemplated pollution control regulations relating tosulfur dioxide emission to the atmosphere as well as those containingvery large amounts of sulfur dioxide. In all such instances, a verysubstantial amount of the sulfur dioxide impurity content of the gasstreams to be treated can be removed therefrom by the process of thepresent invention, the sulfur dioxide impurities thus removed beingrecoverable for nonpolluting disposal or use. The present invention isof particular .value with respect to the stack gas streams from manyindustrial operations, such as the burning of sulfur-bearing fuels, inwhich the gas stream contains from about 0.05 percent to about 10% S0 byvolume. Such gas stream, under the contemplated and desirable pollutioncontrol regulations under consideration, could not simply be vented as anon-polluting discharge to the atmosphere. By means of the presentinvention, however, the sulfur dioxide impurity content of such streamcan be reduced to not more than about 250 ppm, i.e., parts per millionparts of gas by volume. Where even more restrictive pollution controlregulations are deemed necessary or advisable, the process of thepresent invention can be employed to reduce the sulfur dioxide impuritycontent of such gas stream to less than about l00 ppm. In particularembodiments of the invention, the sulfur dioxide impurity concentrationin the gas stream discharged from the absorption zone upon contact withthe specified N-alkyl lactam will be from about 50 ppm to about ppm. Inresponse to the genuine concern for the environmental aspects of variousindustrial operations and the discharge of gas streams therefrom to theatmosphere, the present invention further permits the discharge of gasstreams having sulfur impurity concentrations of as low as from about 5ppm to about 50 ppm, rendering the present invention adaptable to anylikely standard that might reasonably be adapted with respect to sulfurdioxide emissions for non-polluting discharge of gas streams to theatmosphere. As used herein, the term -non-polluting discharge" of gasstreams to the atmosphere refers to the discharge of gas streams to theatmosphere with sulfur dioxide impurity contents of less than those ofapplicable national, state or local pollution control regulations, butnevertheless generally less than about 250 ppm and in any event notexceeding about 300 ppm.

In accordance with the process of the present invention, the selectivesolvent employed, i.e., N-alkyl lactam, is preferably an N-alkylpyrrolidone or piperidone, generally having from about one to about 16carbon atoms in the alkyl group, which includes N-cycloalkyl groups.Most preferred are the Nlower alkyl groups of l to 6 carbon atoms,including N-lower cycloalkyl groups, such as the N-cyclohexyl group. Themost preferred selective N-lower alkyl pyrrolidone for use in thepractice of the present invention comprises N-methyl pyrrolidone.Illustrative of other liquid N-lower alkyl lactams suitable for use assolvents in the practice of the present invention include, but are notnecessarily limited to the following: N-ethyl pyrrolidone, N-propylpyrrolidone, N-isopropyl pyrrolidone, N-t-butyl pyrrolidone, N-n-butylpyrrolidone, N-n-hexyl pyrrolidone, N-cyclohexyl pyrrolidone, N-n-octylpyrrolidone, N- isooctyl pyrrolidone, N-n-decyl pyrrolidone, N-undecylpyrrolidone, N-dodecyl pyrrolidone, N-tetradecyl pyrrolidone,N-hexadecyl pyrrolidone, N-methyl piperidone, N-ethyl piperidone,N-propyl piperidone, N- isopropyl piperidone,-N-t-butyl piperidone,N-n-butyl piperidone, N-n-hexyl piperidone, N-n-octyl piperidone,N-isooctyl piperidone, N-n-decyl piperidone, N- undecyl piperidone,N-dodecyl piperidone, N- tetradecyl piperidone, N-hexadecyl piperidone,etc.

It should be noted again, however, that the preferred selective solventsfor use in accordance with the process of the present invention compriseN-lower alkyl pyrrolidone and N-methyl pyrrolidone in particular.

The N-alkyl pyrrolidones and piperidones employed in accordance with theprocess of the present invention are lactams of the gammaand deltaaminoacid derived from butyric acid, valeric acid and caprylic acid.

'Since such substances are cyclic acid amides, they are neutral andtherefore act in accordance with the pres-. ent invention as physicallydissolving absorbents, such materials having an affinity for theselective removal of sulfur dioxide. Thus, in accordance with thepresent invention it has been discovered that such N-alkyl lactams caneffectively act as a selective solvent for sulfur dioxide from a mixtureof wet and dry gases containing the same, such mixture of gases being aresult, for example, of the burning or combustion of high sulfur fuelsor from the production of sulfur itself. In this regard, one of theadvantages of the use of N-alkyl lactams in accordance with the presentinvention, particularly, N- methyl pyrrolidone, resides in the fact thatsuch solvents are water-soluble. This, therefore, allows sufficientoperation in wet gases and minimizes loss of the solvent material.Moreover, as will be described hereinafter, it allows' for thetoleration of certain amounts of water in the system without decreasingthe effectiveness of the solvent action.

Thus, for example, a stack gas resulting from the combustion of highsulfur containing fuels may contain in addition to the sulfur dioxideresulting from the combustion of the sulfur in the fuel, carbon dioxide,air, nitrogen, water vapor and other miscellaneous gaseous components.It is such typical gaseous mixture containing sulfur dioxide that can beeffectively treated in accordance with the present invention by the useof the N-alkyl lactam as a selective solvent for the sulfur dioxideconstituent. Accordingly, the process of the present invention is ofparticular importance with respect to a wide variety of industrialsystems that require the buming of high sulfur fuels for operation orinvolve the incineration of sulfur compound waste gases. Thus, forexample, the process of the present invention, as will be describedhereinafter, can be effectively employed to eliminate the air pollutionassociated with the utilization of high sulfur containing feed stock orthe burning of high sulfur containing fuels utilized for example, in anelectric power generating station or other industrial systems where S isgenerated and dissipated in stack gases. The expression to eliminate theair pollution associated with the generation and dissipation of S0 instack gases refers, as will be understood from the disclosure above, tothe reduction of sulfur dioxide content of stack gases or otherindustrial waste gases to less than the permissible sulfur dioxideimpurity content for non-polluting discharge" to the atmosphere underapplicable regulatory specifications as set forth above.

The process of the present invention is generally carried out forconvenience in assuring proper contact of the gas stream having sulfurdioxide impurities to be removed therefrom with the N-alkyl lactamsolvent by contacting an ascending stream of stack gases, or other gasesto be treated, containing sulfur dioxide impurities with a descendingstream of the liquid N-alkyl lactam. The N-alkyl lactam, preferablyN-methyl pyrrolidone, may be either water-free or an aqueous solution ofthe N-alkyl lactam containing up to about 25 percent or more water byweight of said solution. Any conventional type of absorption towersuitable for such countercurrent contact of gas and liquid can beemployed for this purpose. Thus, for example, it is possible, inaccordance with the present invention, to contact the rising stream ofgas with the descending stream of N-alkyl lactarn, e.g., N-alkylpyrrolidone, in a conventional bubble-tray tower, baffle tower, packedtower or any other suitable tower construction that permits thecountercurrent flow and contact of the gas and liquid streams involvedin the practice of the present invention.

The remarkable ability of the subject N-a lkyl lactams to strip sulfurdioxide from gas streams, not heretofore appreciated in the art, permitsSO -containing gas streams flowing at commercially significant flowrates to be treated in an absorption zone of practical, commercial sizefor incorporation in conventional industrial processing operations.Thus, the absorption or contacting zone of the present invention, forcontact of the N -alkyl lactam solvent with sulfur dioxideimpuritycontaininggas streams flowing at gas mass flow velocitiesgenerally in the range of from about 200 to about 3,000 lbs/hr/ft, willgenerally range in length from about 5 ft to about ft. It will beappreciated by those skilled in the art that the particular nature andconstruction characteristics of the absorption zone employed in anyparticular embodiment of the invention will, together with otherpertinent operating factors, influence the size of the absorption zoneemployed in that particular embodiment. Thus, the length of theabsorption zone will depend upon, for example, whether the absorptionzone comprises a conventional bubbletray tower or a packed tower or someother tower configuration, as well as upon such factors as the gas flowrate employed, the water content of the N-alkyl lactam solvent and ofthe gas stream being treated, the resulting viscosity of the lactamsolvent, surface tension effects, tray spacing, operating temperatures,solvent recycle rates and the like. It will be" appreciated, of course,that it is highly desirable that the absorption zone-be as-small aspossible while accommodating the volume of gas to be treated andstripping the sulfur dioxide impurities therefrom sufficiently to permitthe non-polluting disposal or other use of the treated gas stream, so asto minimize capital cost and maintenance and operating expenses.

As indicated above, the S0 stripping operation of the present inventionis carried out at gas mass flow ve locities ranging from about 200 toabout 3,000 lbs/hrlft, with rates of from about SOD-1,000 to about 2,000lbs/hr/ft being commonly employed in preferred embodiments of theinvention. In the practice of the invention, the gas flow, i.e., lbs/hr,to the absorption zone and the cross-sectional area of the absorptionzone are determined to provide gas mass flow rates generally within thelimits indicated. Such mass flow rates permit the desired S0 strippingto be accomplished in an absorption zone having the necessary anddesirable equilibrium stages within the commercially feasible overallheight limitations disclosed herein. As the gas mass I flow rates aredecreased below the lower end of the indicated range, thecross-sectional area of the absorption zone may be too large, relativeto the characteristics of the overall processing operation, to avoidchanneling of the gas through the absorption zone such that the gas hasan inadequate opportunity for contact with the lactarn solvent withinthe limits of a feasible height of the absorption zone. On the otherhand, an absorption zone will not be employed with such a small relativecross-sectional area as to requirean excessive column height, resultingin an undesired and excessive pressure drop requirement and expense tomaintain gas flow and in an excessive entrainment of lactam in thetreated gas discharged from the absorption zone.

In general, it has been found that an absorption zone having theequivalent of from about three to about eight theoretical plates orequilibrium stages is suitable and adequate for many sulfur dioxidestripping operations in accordance with the present invention. At trayefficiencies of 30-50 percent and at tray spacings generally of about 2ft, bubble-tray absorption towers having a length of from about to 40 ftcan be employed although somewhat larger bubble-tray tower, up to about85 ft, may be employed to permit flexibility in achieving optimumoperating conditions and high stripping efficiency. As previouslyindicated, it will be understood that the desire to obviate any adverseenvironmental aspects of industrial gas disposal is necessarilyaccompanied by an equal desire to minimize the economic consequencesthereof, so that the absorption zone in any particular application willdesirably tend toward the smallest absorption zone needed to achieve thedesired sulfur dioxide stripping in any given appli-' cation. When apacked column-type absorption zone is employed, e.g., employing l-inchring-type packing or some equivalent packing, it has generally beendetermined that about 1 to about 3 ft of packed column is required foreach theoretical plate or equilibrium stage required for gas-liquidcontact. As a result, a packed column of this type, having a length ofabout 5 to 25 ft, will generally be suitable and adequate although sizesoutside this range can also be employed. The length of the absorptionzone employed, therefore, in any particular application of the inventionwill depend upon a variety of construction and operating conditionspertinent to achieve the degree of sulfur dioxide strippings necessaryor desirable in any given application, but in all instances will bewithin the range of practical commercial feasibility.

By effecting, advantageously, such counter-current flow of the gasstream containing sulfur dioxide impurities and the N-alkyl lactam ofthe present invention, e.g., N-alkyl lactam water solution, particularlyN- rnethyl pyrrolidone, it is thus possible to selectively remove thesulfur dioxide from said gas stream since the descending liquid streamof N-alkyl lactam or watersolution thereof absorbs essentially all ofthe sulfur dioxide in the initial gaseous stream. As a result, thetreated gas leaves the absorption tower essentially free of sulfurdioxide, while a liquid stream of N-alkyl lactam rich in dissolvedsulfur dioxide leaves the bottom of the absorption tower or othersuitable counter-- current gas-liquid contact apparatus. As ishereinafter discussed, the treated gas stream having sulfur dioxidestripped therefrom is, preferably, passed through a water wash sectionto extract all of the N-alkyl lactam, e.g., N-methyl pyrrolidone, fromthe scrubbed gas stream prior to the discharge thereof to the atmosphereor to any other disposal or use of said treated gas stream.

The liquid stream of N-alkyl lactam, particularly N- methyl pyrrolidone,containing dissolved S0 and water can subsequently be stripped of S0 andsome or all of the water content thereof, in a distillation zone, so asto recover the N-alkyl lactam solvent for recycling and reuse andfurther sulfur dioxide absorption operations. The recovered S0 gaseousproducts of said distillation zone, as indicated previously, can besubsequently used, e.g., as in the production of sulfur trioxide, themain component employed in the production of sulfuric acid, or, e.g., toproduce sulfur by the reaction of said S0 with hydrogen sulfide. Thestripping of the dissolved S0 from the liquid selective solvent iseasily effected in a stripping or distillation column or still operatedat a higher temperature and/or lower pressure than that employed for theabsorption of the S0 in the solvent. By operating the stripping columnor still at such an elevated temperature, e.g., about 100C to about200C, the sulfur dioxide gas is easily removed from the lactam andrecovered, while the liquid lactam solvent can be withdrawn from saidstripping column or still for subsequent utilization for the absorptionof sulfur dioxide from additional quantities of stack or similar gasmixtures having sulfur dioxide impurities therein.

It should be noted that the absorption zone or tower operated to absorbsulfur dioxide in the descending selective N-alkyl lactam solvent canadvantageously be operated at ambient temperatures or temperaturesslightly above ambient temperatures. In this regard, it should be notedthat the selective N-alkyl lactam solvent, and aqueous solutions thereofcontaining up to about 25 percent water by weight of said solution ormore, have high absorption capabilities for the selective absorption ofsulfur dioxide such that the absorption tower generally can be operatedat as low a temperature as can economically be obtained. Thus, theabsorption tower in accordance with the present invention can beoperated at gas temperatures approaching as low as about 0C and as highas about 75C. It will be understood that higher gas temperatures in theabsorption zone can be tolerated, but the absorption capabilities of theN-alkyl lactam solvent will begin to sharply decrease as highertemperatures are employed, as witnessed by the desirable stripping ofthe dissolved S0 from the lactam in a regeneration distillation zone orstill as indicated above. As the gas stream to be treated willordinarily be at an elevated temperature, it is generally unnecessaryand undesirable to cool the gas stream to temperatures approaching 0Cprior to passage through the absorption zone. Accordingly, a preferredgas temperature range in the absorption zone is from about 35C to about65C. Gas temperatures of about 35C or slightly lower up to about 50C canbe reached by pre-cooling the gas stream with conventional water and aircooling techniques without the necessity for incurring the economicburden of incorporating a refrigeration system to chill the gas, as isordinarily required when gas temperatures below about 35C are employed.Water cooling can thus be conveniently employed to cool the gas streamto from about 35C or somewhat lower, e.g., 32C, while air coolingtechniques can generally be employed to cool the incoming gas stream toabout 50C. It will be appreciated that somewhat higher gas temperatures,e.g., on the order of about C to about C, can also be advantageouslyemployed within the preferred gas temperature range indicated above. Aswill be discussed in greater detail hereinafter, a gas stream to betreated that is available at an elevated temperature can be utilized, inheat exchange relationship, to supply all or a portion of the heatrequirements of the distillation zone for regenerating N-alkyl lactamsolvent by the stripping of the S0 content therefrom. It should be alsonoted that the pressure employed in the absorption zone is not acritical feature of the invention, it being generally preferred that theabsorption tower be operated at atmospheric or slightly aboveatmospheric pressure, e.g., about 2 or 3 lbs of positive pressure, toassure an adequate flow of the gas being treated through the absorptionzone and ultimately to the stack for discharge to the atmosphere. itwill be understood, however, that there may be instances wherecompression of low pressure SO -bearing gases will be desirable. Theabsorption efficiency of the lactam solvent is increased in suchinstances in that the amount of solvent required is thus reduced. Suchoperation would be particularly advantageous, of course, where thetreated gas stream is desired at an elevated pressure.

It has been discovered that the use of the N-alkyl lactam solvent, i.e.,N-methyl pyrrolidone, in anhydrous form is best or most advantageous forthe desired stripping of sulfur dioxide from the mixture of gasescontaining sulfur dioxide impurities as provided in the practice of thepresent invention. It should be noted, however, that the N-alkyl lactamsolvent can tolerate appreciable quantities of water while neverthelesshaving excellent solvent powers for sulfur dioxide gas. As a result, itis sometimes desirable to have a proportion of water present in theselective N-alkyl lactam solvent, in that the presence of minor amountsof water tends to aid in the stripping of the dissolved sulfur dioxidefrom the selective solvent during regeneration in a distillation zone orstill. In this regard, it should also be noted that some water willgenerally be present in the solvent in the absorption zone in any event,since water will ordinarily be absorbed from the gases being treated inthe absorption zone or column. Completely anhydrous conditions areextremely difficult to maintain, therefore, in many practicalapplications of the invention for the treatment of industrial gasstreams. Based upon the considerations relating to the power andselectivity of the solvent, and the necessity for providing an ease ofstripping during solvent regeneration in accordance with the presentinvention, it has been discovered that the selective N-alkyl lactamsolvent may advantageously contain up to about 25 percent water byweight or more and, in fact that such a water content may be preferredfor most efficient operation of the instant process in variousapplications thereof.

in order to recycle essentially anhydrous lactam solvent from theregeneration zone to the absorption zone, it is necessary to distill thesolvent in the regeneration zone at about 200C to assure the desiredstripping of the S dissolved therein. By operation of the regenerationstill so as to recycle a lactam-water stream to the absorption zone, theheat requirements in the regeneration zone can be appreciably reduced.As the effectiveness of the S0 stripping power of the N-alkyl lactam isnot appreciably diminished, as indicated above, water contents of up toabout 25 percent or even 30 percent, by weight or more can be toleratedin the lactam stream recycled to the absorption zone. As the watercontent of the recycled lactam stream increases, the heat requirementsin the regeneration zone are lowered significantly, providing acorresponding advantage in the overall economics of the process of thepresent invention. Thus, at about 20 percent water by weight in therecycled lactarn stream, the boiling point of the lactam-water mixtureregenerated in the distillation zone is only about 107C.

In some embodiments of the invention, therefore, it has been founddesirable to operate the regeneration zone so as to recycle alactam-rich stream therefrom to the absorption zone with an appreciablewater content. In such embodiments, a water content of about 20 percentto about 25 percent by weight in the lac-tam stream thus withdrawn fromthe regeneration zone has been found particularly advantageous. Whilethe effectiveness of the lactam solvent at this water level is somewhatless effective then such solvents at a lower water level, the heatrequirements for operation of the regeneration zone are appreciablyreduced, as indicated above, and the regeneration zone can be operatedwith little or no lactam carryover with the SO water vapor streamtherefrom, thus simplifying the regenerationreflux operation andminimizing any loss of lactam in said regeneration operation. For mostefficient S0 stripping from the gas stream being treated, however, it isgenerally preferred to operate the regeneration zone so as to recycle alactam stream having a lower water content, providing the absorptionzone with a lactam having a water content of from about 2 percent toabout 10 percent by weight, with from about 5 percent to about 10percent, being particularly preferred in many applications of thisembodiment of the invention. This embodiment, it will be appreciated, isof particular value where a very high degree of stripping is desired andwhere the heat requirements of the regeneration zone can be met withoutimposing any undue economic burden on the overall system. in practical,commercial applications of the invention, the external heat requirementscan be minimized or eliminated as indicated below. The water content ofthe lactam stream recycled to the absorption zone, therefore, can bevaried in the practice of the present invention to provide a desirablebalance between the S0 stripping characteristics of the solvent in theabsorption zone and the heat requirements of the regeneration zone aseffected by the practical considerations that pertain to the particularoverall conditions of a given application of the invention. As indicatedabove, the water content of the recycled lactam stream and thus of thelactam present in the absorption zone will be within the limits ashereinabove indicated.

it will be appreciated that, in general, it would be advantageous andhighly desirable to carry out the process of the present inventionwithout the necessity for supplying heat from any external source forthe operation of the regeneration zone. As indicated above, the abilityto tolerate a small amount of water in the absorption zone provides adesirable reduction in the overall heat requirements of the regenerationzone. As a means for minimizing or even eliminating the need for suchexternal heat, the present invention can advantag'eously be carried outso as to utilize the heat content of the incoming gas stream to betreated as the source of heat for operation of the regeneration zone. Inmany applications of the present invention, the incoming gas to betreated will have such available heat naturally occurring therein, as inhot combustion gases, that must be cooled, in any event, prior topassage through the absorption zone at temperatures within the rangepreviously set forth. Operating expenses can be reduced in a desirablemanner, therefore, by utilizing the excess heat of the gas to betreated, through an appropriate heat exchange medium, to provide thedistillation heat requirements of the regeneration zone.

It will also be appreciated that any additional excess heat in theincoming gas stream, beyond that required for operation of theregeneration zone, may also be recovered by any suitable heat exchangeoperation, as to pre-heat the lactam stream passing to the regenerationzone and to heat the treated gas stream of reduced S content tofacilitate its passage through the discharge stack. In otherapplications of the invention, it may be desirable or necessary for somepurpose to utilize the heat requirements of the incoming gas stream insome other manner. In any circumstance where an excessive amount of heatis otherwise available, it will be understood that the present inventiondoes not require the utilization of the heat available in the incominggas stream to be treated apart from such utilization as will enhance theeconomic aspects of the practice of the present invention. Suchembodiments of the invention in which the heat requirements thereof areminimized, however, will generally be of particular significance since,as previously discussed, the present invention relates to anenvironmental necessity in existing operations with minimum cost desiredso as not to adversely effect or jeopardize the economic justificationfor continuing the basic operation itself. The flexibility existing inthe practice of this invention to achieve optimum operating conditions,together with the ability to regenerate the lactam employed, togetherwith the ability to minimize loss of lactam as hereinafter set forth,all enhance the great commercial significance of the present inventionin air pollution control and environmental protection.

In the present invention, the sulfur dioxide that is removed from theselective N-alkyl lactam solvent in the stripping column of theregeneration zone is withdrawn from the top of such column, togetherwith water vapor and any CO that may be present, and may be introducedinto a further wash column where the sulfur dioxide gas is washed withwater. Such washing of the sulfur dioxide gas will allow further removalof any entrained solvent to that even further solvent can be recoveredfor reuse in accordance with the process of the present invention. Thesulfur dioxide that is obtained from such a washing process can, ofcourse, be compressed for recovery and reuse in any further desiredmanner. It should be noted that the sulfur dioxide obtained by strippingthereof from the gas stream being treated by the selective solventprocess of the present invention is very useful as a reactant forsubsequent production of sulfur or sulfuric acid, both products havinghigh desirability in the art.

The treated gas stream passing from the absorption zone at the indicatedflow rate, with S0: stripped therefrom, will be accompanied byrelatively small amounts of N-alkyl lactam removed from the absorptionzone by the gas stream flowing therethrough. The lactam thus removedfrom the absorption zone may be present in the treated gas stream invaporized form or may be entrained as liquid droplets in the gas stream.While the proportion of lactam thus vaporized or entrained in thetreated gas stream may be relatively small, it will be appreciated thatthe cumulative amount of lactam thus removed from the absorption zone,unless recovered, would represent an operating expense of considerablemagnitude in the practice of the present invention in continuous S0stripping from gas streams in largescale industrial operations. Suchlactam removal in the treated gas stream is increased, at relativelyhigh gas flow rates, high pressure drop across the absorption zone andat relatively high temperature conditions in the absorption zone. At60C, a gas stream passing at a rate of about 1,000 lbs/hr/ft through anabsorption zone containing the preferred N-methyl pyrrolidone liquidsolvent would be discharged therefrom with a lactam content of about17.5 lbslhrlft At lower gas temperatures in the absorption zone, theamount of lactam vaporized or entrained in the discharged gas streamwould be reduced, but would nevertheless represent a highly significantloss of lactam. At 35C, the gas stream discharged from the absorptionzone at about 1,000 lbs/hr/ft would be accompanied by about 4.3lbs/hr/ft of said N-methyl pyrrolidone. For an absorption zone having across-sectional diameter of about 9 ft, i.e., a cross-sectional area ofnearly 64 ft*, a loss of N-methyl pyrrolidone totaling about 6,400 lbsper day would be encountered. This loss, at existing market rates, wouldthus amount to an operating loss of $3,000.00 per day, which on acumulative basis, would constitute an operating loss on the order ofapproximately 1 million dollars a year. Such a loss, it will beappreciated, might well render the remarkable S0 stripping process ofthe present invention economically undesirable despite the highlyadvantageous sulfur dioxide stripping achieved thereby and theaccompanying ability to regenerate the selective solvent employed in aconvenient, economically suitable manner without undue loss ordegradation of the selective solvent employed. In desirable embodimentsof the overall process of the present invention, however, it is readilypossible to recover the lactam vaporized or entrained in the gas streamdischarged from the absorption zone so as to avoid an unacceptable lossof lactam in the treated gas stream discharged to the atmosphere orotherwise disposed of after S0 scrubbing in the absorption zone.

In the practice of the present invention, at least about 90 percent byweight, advantageously at least about 98 percent, preferably 99 percentor more, of the N-alkyl lactam vaporized or entrained in the treated gasstream discharged from the absorption zone may be recovered by passingthe gas stream from the absorption zone directly through awater-containing scrubber zone prior to discharging the gas stream tothe atmosphere or other contemplated disposal means. The treated gasstream will thus pass from the absorption zone to the scrubber zone,conveniently located at the top of the absorber column or column, at theindicated gas mass flow velocity of from about 200 to about 3,000,typically about 1,000 to 2,000, lbs/hr/ft the gas temperature being fromabout 0C to about C, advantageously about 35C to about 65C. Because ofthe relatively small amount of lactam in the gas stream passing from theabsorption zone and in light of the relatively high solubility of thelactam in water, a relatively small amount of water and a small scrubberzone can be employed to minimize the loss of lactam in the treated gasdischarged to the atmosphere. Thus, the amount of water needed in thescrubber zone will generally range from only about 0.1 percent to about5 percent by weight of the lactam employed in the absorption zone, mostcommonly about 0.5 percent to about 1 percent water by weight of lactamin the absorption zone being sufficient.

The scrubber zone may comprise any conventional construction permittingsuitable gas-liquid contact,

e.g., a bubble-tray tower, baffle tower, packed tower or any othersuitable gas-liquid contacting tower or column. in a preferredarrangement in this embodiment of the invention, the treated gas streamfrom the absorption zone passes directly upward into the scrubber zonefor counter-current flow and contact with a descending stream of water.While the size of the scrubber zone will necessarily depend upon variousoperating factors and conditions, such as the particular gas flow rateemployed, the amount of lactam vaporized or entrained therein, the typeof gas-liquid contact construction employed, and the like, the scrubberzone will, in general,

be about one-fifth to about one-third the size of the col umn employedfor the absorption zone. The scrubber zone will thus have a length ofgenerally from about 2 ft to about 25 ft, typically from about 3 ft toabout 15 ft, although sizes outside this range may be employed providingthe gas-liquid contact provided therein is such as to permit the waterin the scrubber zone to remove at least about 90 percent by weight anddesirably essentially all of the lactam accompanying the treated gasstreampassing therethrough at the indicated gas mass flow rates.

During continuous operation utilizing the water scrubber hereinabovedisclosed, water may be fed to the scrubber at the rate that a waterstream having lactam dissolved therein is withdrawn from the scrubber.The water may be fed to the scrubber zone at any convenient temperature,generally within the range of from about C to about 75C, e.g., about35C. The rate of water flow to and from the scrubber zone is relativelylow, as hereinafter indicated, in keeping with the relatively low waterrequirement in general with respect to the desired, essentially completerecovery of lactam vaporized or entrained in the gas stream beingtreated. it is within the scope of the present invention to pass thelactam-containing water stream removed from the scrubber zone eitherdirectly into the absorption zone or, without so passing into theabsorption zone, to the solvent regeneration zone for the recovery ofthe lactam content thereof together with the lactam removed from theabsorption zone for regeneration therein. While, in a relatively smallgas-treating plant, the water stream from the scrubber zone mayadvantageously be fed directly into the absorption zone, it is generallypreferred that the water removed from the scrubber zone be passedinstead to the regeneration zone. It will be understood that thelactamcontaining water stream removed from the scrubber zone may beheated by passage through suitable heat exchangers, as for example toutilize a portion of the excess heat available in the incoming gas to betreated or in the lactam-rich stream recycled from the regeneration zoneto the absorption zone. it should also be noted that the waterrequirements of the scrubber zone are entirely compatible with theoverall ability of the system to tolerate the presence of water with theN-alkyl lactam in the absorption zone as previously discussed. Thus, theprocess of the present invention can be advantageously operated withinthe desired water content limits without any adverse effect resultingfrom the incorporation of the lactam-containing water stream from thescrubber zone with the SO -containing lactam stream removed from theabsorption zone for concurrent treatment in the regeneration zone. Aspreviously indicated, the presence of water within the indicated limitsdoes not unduly effect the remarkable ability of the subject lactam, inthe scrubber zone through which a gas streamhaving vaporized andentrained N-methyl pyrrolidone is passed at the indicated flow rates.The use of such other solvent in place of water, however, is notgenerally preferred as the liquid stream removed from the scrubber zonewith the recovered lactam therein would then need to be passed to aseparate regeneration still for the recovery of the desired lactam. Inthe preferred embodiment, the lactam-containing water stream, as hereinindicated, can be conveniently passed directly to the regeneration zonein which the lactam removed from the absorption zone is regenerated.

The novel process of the present invention is hereinafter furtherdescribed with reference to the accompanying drawing illustrating aschematic flow chart of a' suitable system for carrying out the processof the present invention. As shown, a sulfur dioxide impuritycontaininggas stream is introduced through line 10, blower 11, and line 12 intoheat exchanger 13 wherein it is cooled by a suitable heat exchangemedium in line 15 from the regeneration zone, and, if desired, into heatexchanger 13a for further indirect heat exchange with liquids, as inline 15a, constituting the liquid streams of line 27 passing to still30. Other cooling means, of course, may be employed as necessary ordesired to reduce the gas temperature to the desired levels heretoforeindicated. The cooled gas is introduced through line 16 into the bottomportion of column 20, comprising a lower absorption zone or section 21and an upper scrubber or wash zone or section 22. The gas having S0impurities therein rises upwardly through absorber section 21countercurrent to a descending stream of N-alkyl lactam solvent, i.e.,N-alkyl pyrrolidone introduced into the upper portion of said absorbersection 21 through line 23.

From the top of absorber section 21, the gas stream having S0 strippedtherefrom passes into water wash or scrubber section 22, wherein itcontinues to pass upwardly countercurrent to a descending stream of washwater introduced into the top of said section 22 through line 24. Thewater wash stream in said section 22 serves to recover or remove all ormost of any N- alkyl lactam that is entrained or vaporized in thescrubbed gases entering said scrubber section 22 directly from saidabsorber section 21. The treated gases are removed from the top ofscrubber section 22 through line 25 and may be vented through suitablestacks for nonpolluting discharge to the atmosphere without anunacceptable loss of lactam or may otherwise be disposed of or utilized.If desired, the treated gas stream may be passed in heat exchangerelationship with the incoming gas stream in exchanger 13c to furthercool said incoming stream and to facilitate discharge of the treated gasstream through the stack.

The wash water containing most of the N-alkyl lactam accompanying thegas stream passed from absorber zone 21 to scrubber zone 22 is removedfrom the bottom of section 22 through line 14. if desired, a

portion of the water removed through line 14 may be recirculated throughline 26 and line 24 into the top portion of scrubber section 22.

From the bottom of absorber section 21, a solution of SO; in aqueousN-alkyl lactam is removed through line 27 for passage, if desired,through heat exchanger 130 and/or heat exchanger 28 prior to beingintroduced into distillation column 30 wherein it is further heated byindirect heat exchange with a heating medium, in coil 38 at the bottomof said distillation column 30. Said heating medium, as previouslyindicated, can be preheated as by passage in line 15 through heatexchanger 13. As shown, the lactam-containing water stream removed fromscrubber zone 22 through line 14 may be joined with the lactam streampassing from said absorption section 21 through line 27, if desired. Inanother embodiment of the invention, the water removed from scrubberzone 22 may be passed through line 14a to a cooling zone 13b to cool theincoming gas stream, with a fly ash sludge containing trace amounts ofthe N-alkyl lactam and S being removed from the system through line b.In this embodiment, incoming gas passes through blower 11 and line 12ainto cooler 13b, and the cooled gas stream passes through line 160 intothe bottom of column 20.

From the bottom of distillation column 30, a regenerated N-alkyllactam-rich stream is removed through line 23, passing, for example,through said heat exchanger 28 for indirect heat exchange with the S0containing lactam stream passing in line 27 from scrubber zone 21 todistillation zone 30. From said heat exchanger 28, the lactam-richstream continues through line 23, through heat exchanger for furthercooling by indirect heat exchange with cooling water before beingintroduced into the top portion of absorber section 21 at an appropriatetemperature to permit operation of the absorption zone at the indicatedtemperature levels.

The S0; and water vapor stripped or distilled off from the solution indistillation zone 30, together with any small amount of CO that may bepresent, pass from the top of said distillation zone 30 through line 31.A portion of the water vapor is condensed in partial condenser 32 fromwhich the resulting gas-liquid mixture is passed to reflux accumulator33. Water is recirculated through reflux to said distillation column 30through line 33a. S0 and water vapor from accumulator 33 pass throughline 36 to compressor 37, wherein the gaseous mixture is compressed toabout 100 psig to condense the water portion thereof and is thereinafterpassed through line 39 to cooler 47 and to accumulator 48. From saidaccumulator 48, a recycled vapor stream containing any CO present,together with a small amount of S0 is passed through line 49 for recycleinto absorption zone 21 of column 20 together with additional quantitiesof sulfur dioxide-containing gas entering said absorption zone 21through line 16. An aqueous solution of S0, is passed from accumulator48 through line 50 into a second distillation zone, or sulfur dioxidestill, 40. Live steam is introduced into the bottom of said still 40 toline 41 in order to distill off the $0, from the water. The water thusstripped of S0 is removed from the bottom of still 40 through line 42and can, if desired, be recycled to water scrubber zone 22 through line24.

S0 is removed from the top of still 40 through line 43 at a temperatureof, for example, about 110F and is condensed in condenser 44 operated ata pressure of about psi(g) and a temperature from about to F. Thecondensate from condenser 44 is passed through line 46 to liquid S0accumulator 51, from which a portion of the condensed S0, isrecirculated through line 45 into the top of still 40 for reflux. Ifdesired, a recycle vapor stream can be removed from accumulator 51through line 53 for recycle, with vapor in line 49, to absorption zone21. The remainder of the S0 can be removed from said accumulator 51through line 52 as an essentially pure liquid S0 product, having an S0,content of generally from about 90 percent to about percent by weight,containing very small amounts of impurities, mainly CO This liquid S0product represents a valuable by-product of the S0 stripping process,said liquid S0 product being in convenient form for non-pollutinghandling, transport, storage and use.

EXAMPLE 1 In the application of the sulfur dioxide stripping process ashereinabove set forth with respect to the flue gas from an electricpower station having a generating capacity of 250,000 KW, burning a highsulfur bituminous coal, the flue gas having sulfur dioxide impuritytherein is passed at a rate of 26.6Xl0 std. cu ft/hr, or 2X10 lbs/hr,and a temperature of about 350"F through line 10, blower 11 and line 12ainto a cooler 13b where it is washed, or cooled and washed, by a sprayof water introduced into the top of the cooler through line 14a. Fromthe bottom of cooler 13b, a fly ash sludge,.containing trace amounts ofN-methyl pyrrolidone and S0 is removed from the system through line 15b.The cooled and washed flue gases from cooler 13b pass through line 16ato absorber zone 21 of column 20 at a temperature of about F. Thecomposition of the flue gases entering the cooler through line 12a, inpercent by volume, is approximately: $0, 0.2%, CO 11.6%, 0 6.6%, N 79.0%and H 0 3.0%. The composition of the gases removed from the coolerthrough line 16, in percent by volume, is approximately as follows: S00.19%, CO 10.92%, 0 6.14%, N 73.30%, and N-methyl pyrrolidone solvent0.35 percent.

The cooled flue gases pass through the lower absorber section 21 ofcolumn 20 countercurrent to a descending stream of N-methyl pyrrolidonesolvent introduced into the upper portion of said absorber section 21through line 23. The gas stream passing through absorber section 21 isat a temperature of about 95F and has a flow velocity of 2.5Xl0 lbs/hr,with a gas mass flow velocity of about 1,500 lbs/hr/ft.

The scrubbed gas stream passing upward from the top of absorber section21 has its S0 content stripped to approximately 0.02 percent by volume,i.e., 200 ppm. The gas stream passes from absorber section 21 into waterwash or scrubber section 22 of column 20 at the indicated flow rate forcontinued upward passage therethrough countercurrent to a descendingstream of wash water introduced into the top of scrubber section 22through line 24 at a temperature of about 95F and at a flow rate of155,000 lbs/hr. Most of the N-methyl pyrrolidone vaporized or entrainedin the scrubbed gases passing from absorber section 21 to scrubbersection 22 is recovered by the water therein. The washed gases removedfrom the top of scrubber section 22, therefore, not only have SO,stripped therefrom suf'ficiently to permit non-polluting discharge ofthe gas stream to the atmosphere with a greatly reduced, and

TABLE This column stripped away excess water, sulfur dioxide, a smallamount of carbon dioxide and inert gases into a reflux condenser thatseparated a very small recycle vapor before feeding the excess reflux toa small fractionator. The solvent employed in carrying out theseexperiments was N-methyl pyrrolidone containing water in an amount offrom about 10 percent to about 35 percent by weight. The following tableillustrates the resulm of the test conducted as above, indicating in thelast column thereof, the percent of sulfur dioxide absorbed from the gasstream by the N-methyl pyrrolidone solvent:

Temperatures Std. cu. solu- M01 it. gas Percent tion clrpercent mols.mols K Percent Inlet F. avg. flow per H1O in culated/ S02 in liq.[hr.vapor/hr a gas Reboller absorber minute solvent hr. inlet L =Y/X VKabsorbed lator 33 are compressed to a pressure of about 100 psig Y incompressor 37 and are cooled to about 8090F in cooler 47 before enteringthe second distillation zone, namely column 40, at a rate of about35,000 lbs/hr through line 51. Water stripped of S0 is removed from thebottom of still 40 through line 42 at a temperature of about 325F and arate of about 154,000 lbs/hr.

S0 is removed from the top of still 40 through line 43 at a temperatureof about llOF and is condensed in condenser 44 at about 8090F at 75psig. A portion of the liquid 50, thereby obtained is recirculated tostill 40 for reflux, and the remainder is removed to storage asessentially pure liquid S0 having a purity of 90-95 percent, throughline 53.

EXAMPLE 2 The process of the present invention was carried out in pilotplant apparatus substantially as shown in the drawing with sulfurdioxide impurities in flue gases being stripped by means of N-methylpyrrolidone from a stack gas composition containing said sulfur dioxide,air. carbon dioxide, nitrogen and water vapor. Various runs wereconducted with the inlet gas temperature varying from 975 to 1,225F, theaverage temperature of the absorber varying from 75 to 90F. In suchpilot plant runs, the flue gases were introduced into the absorberwithout previous cooling.

The absorber utilized in carrying out these experiments was a 4 inchinside diameter absorber containing 15 inches of inch glass Raschig ringpacking, providing four calculated theoretical plates. Provisions weremade for passing a lactam stream having SO, dissolved therein from theabsorber to a stripping column comprising a 2 inches inside diameterstripping column mounted on top of a main reboiler, such columncontaining 15 inches of /i/ring packing below the feed and a smallamount of stainless steel gauze above the feed.

As can readily be seen from the above table, all of the experimentalruns conducted in the pilot apparatus in accordance with the process ofthe present invention allow for a very effective absorption andstripping of the sulfur dioxide from the stack gas containing the same.It should be noted, however, that where the water content of theN-methyl pyrrolidone solvent was greater than about 25 percent, asomewhat lower percentage absorption was observed. Ease of operation andselectivity of the solvent for the sulfur dioxide indicate that themoisture content, or water content of the solvent should be generallyless than about 25 percent, as under the conditions of the experimentalrun at a range of about 15 to 20 percent by weight.

Again, it is pointed out that the above experiment clearly indicatesthat the N-alkyl lactam is capable of selectively absorbing andstripping sulfur dioxide from a mixture of gases containing the same.This is of particular importance in that the reduction of sulfur dioxidecontaminants in the air is now extremely essential in light of the greatconcern and growing awareness about air pollution and the necessity forenvironmental controls. By employing the process of the presentinvention, an industrial complex engaged in the combus tion of highsulfur-containing fuels can accommodate the concern about air pollutionby assuring that the sulfur dioxide contaminants of waste gas streamsare stripped therefrom so as not to be discharged to the atmosphere inquantities considered to be of a polluting nature under the applicableair pollution control regulations.

EXAMPLE 3 Runs essentially as in Example 1 in which the N- methylpyrrolidone solvent is replaced with substantially equivalent amounts ofthe N-alkyl lactams set forth below provide substantially equivalentresults, i.e substantially equivalent selective absorption and strippingof sulfur dioxide from a gas stream being treated. illustrativeadditional lactams include: N-ethyl pyrrolidone, N-p-butyl pyrrolidone,N-isooctyl pyrrolidone, and N-isopropyl piperidone. The employment ofsuch additional N-alkyl lactams further illustrates the selectivity ofthe process of the present invention for stripping sulfur dioxide from amixture of gases containing the same, and thus illustrates the greatimportance of the process of the present invention with respect to theelimination of the air pollution problems associated with the disposalof industrial gas streams containing sulfur dioxide.

The commercial significance of the present invention is furtherillustrated in the following examples of various embodiments of theinvention carried out in accordance with the description of theinvention as hereinabove set forth and as illustrated in theaccompanying drawing.

EXAMPLE 4 A roaster gas stream from a sulfur smelting operation has acomposition essentially of 84 percent nitrogen, 8 percent oxygen and 8percent sulfur dioxide, expressed as percent by volume. This gas streamis cooled from about 160C to about 65C by heat exchange with the heatingmedium for the lactam regeneration still and with the lactam streamhaving S dissolved therein passing from the absorption zone to thelactam regeneration zone. The cooled gas is passed to the absorptionzone, e.g., section 21 of column 20 of the drawing, at a gas mass flowvelocity of about 1,500 lbS/hr/ft The absorption zone comprises a packedcolumn 12 ft in height packed with Raschag rings and similar ring-typecontact surfaces. The packed column contains liquid N-methyl pyrrolidonesolvent passing downwardly in the packed absorption zone forcountercurrent contact with the ascending gas stream. Regenerated lactamis recycled to the column through line 23, and a lactam stream having S0dissolved therein is withdrawn through line 27. A recycle rate ofregenerated lactam of about 24 gals of lactam per MCF of gas gassingthrough the absorption zone is maintained. A percent water content byweight is maintained in the lactamrich stream fed from distillation zone30 to absorption zone 21 by operating the distillation zone at about150C and 5 psig. The water in scrubber zone 22, into which the gasstream passes directly from absorption zone 21, is maintained at about65C. The scrubber zone comprises 4 ft of packing, i.e., the samering-type contact packing surfaces as employed in the absorption zone.Water is fed to said scrubber zone 22 through line 24 at the rate ofgal/MCF of treated gas passing therethrough. A water stream havinglactam contained therein is withdrawn from scrubber zone 22 through line20 for passage, together with the lactam stream in line 27, todistillation zone 30. The treated gas passing from column 20 fordischarge to the atmosphere is found to have about 99.4 percent byvolume of its sulfur dioxide impurity contents scrubbed therefrom, thedischarged gas stream having an S0 content of about 500 ppm. Theappreciable quantities of S0 thus removed from the gas stream anddistilled off in distillation zone 30 can thereupon be recovered inaqueous solution and can be distilled therefrom, if desired, to obtainan essentially pure liquid S0 product. The treated gas stream, having avery substantial portion of its S0 content stripped therefrom, can befurther treated, if desired, for non-polluting discharge to theatmosphere or can otherwise be processed in any desired manner. Thetreated gas stream upon leaving the scrubber zone will have about 99percent of the lactam accompanying said stream upon passage into thescrubber zone removed therein, so that an unacceptable loss of lactam inthe discharged gas stream is avoided.

EXAMPLE 5 A stack gas stream resulting from the combustion ofsulfur-containing fuel, is processed in a manner centrally as set forthin Example 4. The incoming stack gas to be treated in accordance withthe present invention has a composition, expressed in percent by volume,as follows: CO 15%, H O 8.2%, 0 1.8%, $0, 0.5%, with the balancecomprising nitrogen. This gas stream, cooled to 55C, is passed into anabsorption zone having a bubble plate construction with 2 ft platespacing and six actual contact trays or plates, with a plate efficiencyof about 30-40 percent. The gas mass flow velocity through theabsorption zone is 1,300 lbs/hr/ft The thus treated stack gas passesdirectly from absorption section 21 into scrubber section 22 of thegasliquid contact column, wherein the gas is scrubbed with water in atwo-plate scrubber zone with said 2 ft spacmg.

N-methyl pyrrolidone is employed as the N-alkyl lactam solvent in theabsorption zone, said lactam having a water content of about 5 percentby weight. Regenerated lactam is recycled to said absorption zone at arate of 8.7 gals/MCF, i.e., per 1,000 ft", of gas being treated. Thewater flow rate in the scrubber zone maintained at about 0.25 gal perMCF of gas passing therethrough. Ninety-nine percent of the S0 impuritycontent of the stack gas is stripped therefrom so that the treated gasstream discharged to the atmosphere contains only 50 ppm of S0Essentially all of the lactam vaporized or entrained in the stack gaspassing from the absorption zone into the scrubber zone is recoveredtherein so as to minimize the amount of lactam lost in the dischargedgas stream.

EXAMPLE 6 A gas stream having an S0 content of about 0.05 percent istreated as in Example 5 to strip about 99.0 percent of the S0 impuritiestherefrom. Upon passing through the scrubber zone for contact with watertherein, the treated gas stream is permitted to pass in non-pollutingdischarge to the atmosphere with an S0, content of only 5 ppm. In thisgas treating operation, the gas temperature in the absorption zone is45C, and the gas mass flow velocity is 1,500 lbs/hr/ft". The absorptionzone and the scrubber zone employed comprise the lower and upperportions of a packed column, the absorption zone being 6 ft in heightand the scrubber zone 2 ft in height. A solvent recycle rate of 8.5gals/MCF of gas being treated, the recycled lactam to the absorptionzone having a water content of about 5 percent. The water flow rate tothe scrubber zone is maintained at about 0.2 gal/MCF of gas passingthrough the treating column. Over 98 percent by weight of the vaporizedand entrained lactam is removed from the gas stream during passagethrough the scrubber zone.

EXAMPLE 7 A roaster gas stream, such as in Example 4, has about 99.8percent of its sulfur dioxide impurity content stripped therefrom uponcontact, at about 60C, with N-alkyl lactam, i.e., N-methyl pyrrolidone,in a bubble tray absorption zone. A total of eight actual trays isemployed with a 2 ft spacing, the tray efficiency being about 3M0percent. Three actual trays with said 2 ft spacing are employed in thescrubber zone. The gas mass flow velocity through the absorption zoneand through the water scrubber zone is 1,250 lbs/hr/ft A solvent recyclerate of 9.5 gals/MC'F of treated gas is maintained in the absorptionzone, the water flow rate in the scrubber zone being 0.2 gal/MCF oftreated gas.

The water content of the solvent in the absorption zone is about 4.7percent by weight. The roaster gas has its SO, content reduced,therefore, some 8 percent to 100 ppm in the stripping operation of thepresent invention. Minimal loss of lactam in the discharged gas streamis sustained, the water in the scrubber zone recovering over 99 percentof the lactam vaporized or entrained in the gas stream passingtherethrough, the lactam thus removed from the scrubber zone in thewater stream passing therefrom being regenerated in the lactamdistillation zone for recycle to the absorption zone.

EXAMPLE 8 A combustion stack gas having an S impurity content of about0.5 percent by volume is treated by contact with N-methyl pyrrolidone ina packed absorption column to remove about 99.9 percent of the S0impurities, the treated gas stream being discharged to the atmospherewith an S0 content of 5 ppm. For this essentially complete stripping ofthe S0 impurities, the packed column employed has a height of about ft,and the gas mass flow velocity therethrough is maintained at 1,500lbs/hr/ft The temperature of the gas passing through the absorption zoneis 65C, rich lactam being passed therefrom to a distillation zone forheating therein under reflux at about 1 10C. A regenerated lactam streamhaving percent water by weight is cooled and recycled to the absorptionzone at the rate of 7.2 gals/MCF of stack gas passing through theabsorption zone. The scrubber zone comprises a packed column about 4 ftin height. Essentially all of the vaporized and entrained lactam in thetreated gas passing upwardly from the absorption zone is removed fromthe gas by the water in the scrubber zone, said water, at 0.25 gal/MCFof stack gas, being passed to the distillation zone for recovery of thelactam content thereof.

In further illustrative examples of the remarkable SO, strippingobtainable in the practice of the invention, other N-alkyl lactams ofthe type indicated above can be employed in place of the generallypreferred N- methyl pyrrolidone. Higher molecular weight N-alkylsolvents are, of course, higher boiling and less volatile than N-methylpyrrolidone. As noted above, however, N-methyl pyrrolidone has arelatively low vapor pressure and can be regenerated substantiallywithout loss in the distillation zone. As the lactam vaporized orentrained in the treated gas can, in addition, be substantiallycompletely recovered in the scrubber zone, no significant advantage isobtained in this regard in the use of other N-alkyl lactams in place ofN-methyl pyrrolidone. As the S0 stripping power of N-methyl pyrrolidoneis particularly impressive, the use of N-methyl pyrrolidone is generallypreferred in the practice of the invention, although, of course, otherindicated alkyl lactams can also be employed. When utilizing N-cyclohexyl pyrrolidone for the S0 stripping of the invention, it isgenerally desirable to employ a somewhat faster solvent recycle rate anda lower water content than would be employed in a similar applicationutilizing N-methyl pyrrolidone as the S0 stripping solvent. The solventrecycle rate for N-cyclohexyl pyrrolidone will, advantageously be fromabout 1.5 to about 1.75, e.g., about 1.7, times the recycle rate forN-methyl pyrrolidone. While water can be tolerated in the absorptionzone within the rather wide limits indicated, the water content willadvantageously be from about 2 percent to about 5 percent, preferablyabout 3 percent, in many applications where a very high degree of strip-24 ping is desired utilizing N-cyclohexyl pyrrolidone as the strippingsolvent. As the process of the invention is operable over wide limits ofS0 impurity content, degree of stripping, temperatures and otherconditions, however, it will be understood that the optimum conditionsapplicable in any given application will depend upon the particularconditions pertaining to that application. Various other processingmodifications can be made to achieve optimum operating conditions. Forexample,

excess heatavailable in the incoming gas stream can also be employed tosupply heat for operation of the second distillation zone, i.e., S0still 40.

In continuous gas treating operations, lactam having 80,, dissolvedtherein will be withdrawn from the absorption zone on a generallycontinuous basis with regenerated lactam being recycled from thedistillation zone to the absorption zone at a rate generally within arange on the order of about 5 to about 40 gallons of lactam solvent perMCF of gas being treated. It will be appreciated that the effectivenessof S0 stripping is generally increased by increasing the solvent recyclerate. When a water scrubber zone of the type indicated is employed tominimize loss of lactam in the discharged gas stream, the water streamcontaining dissolved lactam is withdrawn from the scrubber and a waterstream is fed to the scrubber also generally on a continuous basis, therate being much less than that of solvent recycle as the waterrequirements for minim izin g loss of lactam are relatively small. Awater flow rate generally on the order of about 0.05 to about 0.25gallons per MCF of gas being treated is sufficient in most applicationsprovided that an appropriate size scrubber zone is provided. It iswithin the scope of the invention, however, to provide for larger waterflow rates with rates generally ranging from about 0.05 to about 2.0gals per MCF of gas treated being sufficient and satisfactory for thedesired lactam recovery. As previously indicated, however, theparticular processing limitations employed in any given application willdepend upon the degree of S0 stripping necessary or desired in anyparticular application and the particular operating conditions andmaterials employed in that application. In this regard, it should alsobe noted that, in some applications, a gas stream being treated in theabsorption zone will be subject to more than one such strippingoperation as described above to achieve a desired ultimate level of S0stripping. If practical, a somewhat larger absorption zone or mass flowrates outside the precise limits herein set forth may be employed toachieve sufficient S0 stripping for non-polluting discharge to theatmosphere or other disposal or use of the treated gas stream. In otherapplications, a gas stream may have a relatively low SO, content, e.g.,150-300 ppm, that is nevertheless to be reduced to an applicable airpollution control specification of, for example, ppm. Likewise, a gasstream may have an 80, content of about 500 ppm that is to be reduced inthe absorption zone to about 250 ppm for non polluting disposal, anefficiency of stripping in said absorption zone of only about 50 percentbeing required in this instance. Although the present invention iscapable of a highly remarkable and unexpected degree of S0 stripping, asherein indicated, it is also within the scope of the invention to treatsuch gas streams under such less stringent requirements of recycle rate,temperature, water content and the like as to achieve the more limitedS0 stripping required for conformance with the applicable standards eventhough the invention could be utilized to achieve a higher degree of S0stripping if more restrictive standards or an overriding environmentalconcern were to compel such a greater utilization of the invention.

Economic considerations will, of course, ordinarily encourage thepractice of the invention with minimum capital costs, and lactam, waterand utility requirements. It will be understood, however, that incontinuous commercial operations where environmental concerns andregulatory requirements make necessary the continuing adherence toemission control standards without, at the same time, undesiredinterruption of normal operations, a prudent safety factor willordinarily be employed to assure successful, continuous operations.Minor mechanical problems causing less than desired pumping or heatexchange capability, and the like, are thus capable of being toleratedwithout the necessity for costly, unscheduled shut-downs or turnoversinterfering with efficient and continuous gas treating operations. Inthe practice of the invention, therefore, an excess number ofequilibrium stages, e.g., bubble trays or length of packed column, willordinarily be employed over and above that suggested by the theoreticalplates requirements, tray efficiencies and the like as discussed above.For appropriate safety factor purposes, the number of actual trays orheight of column employed, particularly in the absorption zone, may beincreased by up to about 50 percent excess or more, even up to 100percent, to assure adequate stripping on a continuous basis despiteperipheral operating handicaps that may occasionally be encountered. Itis a measure of the striking efficiency of the stripping capabilitiesachieved in the practice of the invention that such ample safety factorscan be employed while remaining within the practical, commerciallyfeasible size and operating limitations disclosed and claimed herein.Accordingly, bubble tray absorption zones in continuous commercialoperations under the invention will often be at least about ft inlength, and in many practical applications, at least about 40 ft inlength, because of such practical considerations for successfulcontinuous operations of a commercial plant. It is also within the scopeof the invention to practice the present invention by passing the gasstream to be treated through two or more separate absorption zonecolumns or towers, the overall height of the combination of absorptionzone columns being generally within the limits hereinabove set forthwith respect to a single such column. In this embodiment, rich. lactamsolvent having S0 dissolved therein can be withdrawn from the bottomportion of each column, with fresh regenerated lactam being introducedinto the upper portion thereof for descending passage in counter-currentcontact with the ascending stream of gas to be treated. Altemately, arich lactam stream having S0 dissolved therein can be withdrawn from thebottom of one such absorption column and introduced into the upperportion of the next column of a series of two or more such separatecolumns. In this embodiment, the rich lactam stream removed from thelast of such a series of columns would be passed to the distillationzone for regeneration of the lactam solvent by removal of the S0dissolved therein. In another embodiment, a single absorption zonecolumn may have regenerated solvent introduced not only at the upperportion as described above, but at one or more intermediate points alongthe column height. It is also within the scope of the invention toenhance the efficiency of the stripping operation by employing anabsorption zone of given, practical size, with two, three or moreseparate stages each with its own separate lactam solvent introductionpoint and draw-off point for lactam having S0 dissolved therein. In thisembodiment, the separate lactam streams withdrawn from each stage can befed to a common distillation or regeneration zone. Likewise, thelactam-rich stream from said distillation zone can be recycled to eachof the separate solvent introduction points on the column. It will beunderstood that each stage of the overall zone or column need not beoperated for maximum S0 stripping efficiency, but that the flexibilityof the invention is further enhanced in meeting any desired degree ofoverall S0 stripping. For example, an absorption zone having an overallheight within the practical, commercially feasible limit recited hereinmay have three separate draw-off sections. If the first two stages werethus operated to achieve 90 percent SO stripping and the third stage,treating a gas stream having a very greatly reduced S0 content, wereoperated at only percent removal, the overall treatment would effect a99.7 percent removal of the S0 content of the gas stream passing throughthe absorption zone. This embodiment, therefore, enhances the readyachievement of extremely high levels of S0 removal in accordance withthe present invention as disclosed and claimed herein.

The present invention thus provides a highly effective, commerciallyattractive technique for stripping S0 from gas streams. The invention isnot only compatible with desired or prospective S0 specifications forair pollution control, but offers a genuine opportunity for loweringsuch specifications and enhancing the recovery of S0 from gas streamsdischarged to the atmosphere. In addition to the high degree of S0stripping achieved in the practice of the invention, over an acceptablerange of operating conditions, the sharp decrease in S0 solubility withtemperature permits a ready regeneration of the N-alkyl lactam forrecycle and reuse. As the N-alkyl lactam solvent can readily withstandthe operating temperatures required in the regeneration operationwithout degradation and the loss of solvent in the overall operation canbe minimized, the present invention is of great importance as a meansfor overcoming the problem of sulfur dioxide emissions resulting fromindustrial gas disposal. The attractiveness of the invention is enhancedby the ability to recover the stripped SO, in a usable form,particularly in the form of an essentially pure liquid S0 product. Thelatter product, in convenient form for non-polluting handling, storageand use, provides a de-' sirable by-product of the S0 strippingoperation, capable of use in the low cost production of sulfuric acidand other sulfur containing products.

A highly significant feature of the present invention resides in itsinherent flexibility as shown above. This flexibility enables theinvention to be utilized to readily satisfy present or prospective SO'emission standards and to further meet even more restrictive standardsas they become applicable without the necessity for a basic redesign ofthe emission control system. The present invention also enables dualpollution control standards, presently contemplated, to be satisfied.Such standards, for example, would require not only that a certain S0ppm level be reached for atmospheric discharge, but that a high degreeof stripping consistent with available technology be achieved. A gasstream having about 500 ppm, under such standards, might be required notonly to be stripped to 250 ppm as an upped, generally applicable level,but might also be required to have at least about 90 percent, or 95percent or higher S removal prior to discharge to the atmosphere. Thepresent invention provides a highly attractive technique for meetingsuch dual standards, with continuous operation of an existing unit beingcapable of achieving increased levels of stripping, as by increasedsolvent recycle rates, without being outmoded by more restrictivepollution control standards adopted after the installation of such aunit. As the stripping of additional quantities of S0 from a gas streamalready containing a relatively low S0 content is generally known to bemore difficult than the same percentage removal at a higher S0 level, itwill be appreciated that the present invention is of major importancewith respect to a problem of immediate and urgent concern throughout theindustrial world.

It will be appreciated that various changes and modifications can bemade in the overall process herein disclosed without departing from thescope of the invention as hereinafter set forth in the appended claims.

Therefore, we claim:

1. A process for treating a gas stream containing sulfur dioxideimpurities to enable said stream to be discharged to the atmospherewithout undersired sulfur contamination thereof comprising:

a. contacting a gas stream containing from about 0.05 percent to aboutS0 by volume with a liquid N-alkyl lactam solvent in a contacting zoneat a gas temperature of from about 0C to about 75C at a pressureassuring the flow of said gas stream through the contacting zone at agas mass flow velocity of from about 200 to about 3,000 lbslhrlft thecontacting zone having a length of from about 5 ft. to about 85 ft., and

b. discharging the thus-treated gas stream to the atmosphere, saiddischarged stream having a sulfur dioxide impurity content of not morethan about 250 ppm, whereby the N-alkyl lactam washes said sulfurdioxide impurities from the gas stream sufficiently to permit thenon-polluting discharge of said gas stream to the atmosphere, saidsulfur dioxide impurities absorbed in said N-alltyl lactam beingconveniently available for nonpolluting disposal or use.

2. The process of claim 1 in which said sulfur dioxide impurities in thedischarged gas stream comprise less than about 100 ppm.

3. The process of claim 2 in which said sulfur dioxide impurityconcentration in the discharged gas stream is from about 50 ppm to about100 ppm.

4. The process of claim 2 in which said sulfur dioxide impurityconcentration in the discharged gas stream is from about 5 ppm to about50 ppm. 1

5. The process of claim 1 in which said gas temperature is from about Cto about 65C.

6. The process of claim 5 in which the sulfur dioxide content of theuntreated gas stream is from about 0.1 percent to about 8 percent byvolume.

7. The process of claim 6 in which said sulfur dioxide content of theuntreated gas stream is from about 0.1 percent to about 0.5 percent byvolume.

8. The process of claim 7 in which the sulfur dioxide impurities in thedischarged gas stream comprise less than about 100 ppm.

9. The process of claim 6 in which said contacting zone has a length ofat least about 10 ft.

10. The process of claim 5 in which said lactam is N- methylpyrrolidone.

1 l. The process of claim 5 in which said lactam is N- cyclohexylpyrrolidone.

12. The process of claim 5 in which said liquid lactam comprises anaqueous solution thereof.

13. The process of claim 12 in which said aqueous solution contains fromabout 2 percent to about 25 percent water by weight.

14. The process of claim 13 in which said water content is from about 15percent to about 25 percent by weight of said lactam solution.

15. The process of claim 13 in which said water content is from about 2percent to about 10 percent by weight.

16. The process of claim 5 in which the sulfur dioxide content of theuntreated gas stream is from about 0.1 percent to about 8 percent byvolume, the sulfur dioxide impurities in the discharged gas streamcomprising less than about 100 ppm, said contacting zone being at leastabout 10 ft. in length.

17. The process of claim 16 in which said N-alkyl lactam comprisesN-methyl pyrrolidone.

18. The process of claim 1 and including removing the N-alkyl lactamhaving sulfur dioxide absorbed therein from the contacting zone andheating to from about 100C to about 200C to strip saidsulfur-dioxide-containing gases therefrom and thereafter recycling saidN-alkyl lactam to said contacting zone for contact with additionalquantities of said sulfur dioxide contaminated gas stream, the strippedsulfur dioxide gases being utilized in the production ofsulfur-containing compounds for subsequent use or non-pollutingdisposal.

19. The process of claim 18 in which said lactam is an N-lower alkylpyrrolidone.

20. The process of claim 19 in which said lactam comprises N-methylpyrrolidone.

21. The process of ciaim 20 in which said gas temperature is from about35C to about 65C.

22. The process of claim 21 in which said sulfur dioxide-contaminatedgas stream is at an elevated temperature above about 65C and includingcooling said gas stream to from about 35C to about 65C prior to contactthereof with said liquid lactam, said gas mass flow velocity being fromabout 1,000 to about 2,000 lbslhrlft 23. The process of claim 22 inwhich the sulfur dioxide concentration in the treated gas streamdischarged to the atmosphere is from about 50 ppm to about 100 ppm, saidcontacting zone being at least about 20 ft. in length, said lactamcomprising an aqueous solution thereof containing up to about 25 percentwater by weight of solution.

24. The process of claim 23 in which said lactam comprises an aqueoussolution thereof containing from about 2 percent to about 10 percentwater by weight of solution.

25. A process for treating a gas stream containing sulfur dioxideimpurities comprising:

a. contacting an incoming gas stream containing from about 0.01 percentto about 50 percent sulfur dioxide by volume with a liquid N-alkyllactam solvent in a contacting zone at a gas temperature of from about0C to about C at a pressure assuring the g flow of said gas streamthrough the contacting zone at a gas mass flow velocity of from about200 to about 3,000 lbslhrlft the contacting zone having a length of fromabout ft. to about 85 ft.; and b. discharging the thus-treated gasstream from the contacting zone, said stream having a sulfur dioxidecontent of not more than about percent by volume of the sulfur dioxidecontent of the un-' treated gas stream, at least about 90 percent byvolume of said sulfur dioxide impurities of the un- -treated gas streambeing removed therefrom by absorption in said N-alkyl lactam, wherebythe sulfur dioxide impurities are very substantially washed from the gasstream containing same, the sulfur dioxide impurities being convenientlyavailable for non-polluting disposal or use.

26. The process of claim 25 in which at least about 95 percent of thesulfur dioxide impurities are removed from the incoming gas stream byabsorption in said N- alkyl lactam, said gas temperature being fromabout 35C to about 65C.

27. The process of claim 26 in which at least about 98 percent of thesulfur dioxide impurities are removed from the incoming gas stream byabsorption in said N- alkyl lactam.

28. The process of claim 25 in which said sulfur dioxide content of theuntreated gas stream is from about 0.1 percent to about 0.5 percent byvolume, at least about 95 percent of the sulfur dioxide impurities beingremoved from the incoming gas stream by absorption in said N-alkyllactam, said gas temperature being from about 35C to about 65C.

29. The process of claim 28 in which said lactam comprises N-methylpyrrolidone containing up to about 25 percent water by weight.

30. The process of claim 29 in which said aqueous solution contains fromabout 2 percent to about 10 percent water by weight.

31. The process of claim 25 in which said sulfur dioxide-containing gasstream is at a temperature above about 65C and including cooling saidgas stream to from about 35C to about 65C prior to contact thereof withsaid liquid lactam and including heating lactam removed from thecontacting zone to from about 100C to about 200C to strip said sulfurdioxide-containing gases therefrom and thereafter recycling said N-alkyllactam to said contacting zone for contact with additional quantities ofsaid sulfur dioxide contaminated gas stream, the stripped sulfur dioxidegases being thereafter compressed and recovered for non-pollutingdisposal or use.

32. The process of claim 31 in which said contacting zone has a lengthof at least about 20 ft.

33. The process of claim 32 in which said lactam comprises N-cyclohexylpyrrolidone.

34. The process of claim 32 in which said lactam comprises N-methylpyrrolidone, said lactam removing at least about percent by volume ofthe sulfur dioxide impurities contained in the incoming gas stream.

35. The process of claim 34 in which said lactam removes at least about98 percent by volume of the sulfur dioxide impurities contained in theincoming gas stream, said absorption zone being at least about 40 ft. inlength.

36. The process of claim 35 in which said gas mass flow velocity is fromabout 1,000 to about 2,000 lbs/hr/ft

2. The process of claim 1 in which said sulfur dioxide impurities in thedischarged gas stream comprise less than about 100 ppm.
 3. The processof claim 2 in which said sulfur dioxide impurity concentration in thedischarged gas stream is from about 50 ppm to about 100 ppm.
 4. Theprocess of claim 2 in which said sulfur dioxide impurity concentrationin the discharged gas stream is from about 5 ppm to about 50 ppm.
 5. Theprocess of claim 1 in which said gas temperature is from about 35*C toabout 65*C.
 6. The process of claim 5 in which the sulfur dioxidecontent of the untreated gas stream is from about 0.1 percent to about 8percent by volume.
 7. The process of claim 6 in which said sulfurdioxide content of the untreated gas stream is from about 0.1 percent toabout 0.5 percent by volume.
 8. The process of claim 7 in which thesulfur dioxide impurities in the discharged gas stream comprise lessthan about 100 ppm.
 9. The process of claIm 6 in which said contactingzone has a length of at least about 10 ft.
 10. The process of claim 5 inwhich said lactam is N-methyl pyrrolidone.
 11. The process of claim 5 inwhich said lactam is N-cyclohexyl pyrrolidone.
 12. The process of claim5 in which said liquid lactam comprises an aqueous solution thereof. 13.The process of claim 12 in which said aqueous solution contains fromabout 2 percent to about 25 percent water by weight.
 14. The process ofclaim 13 in which said water content is from about 15 percent to about25 percent by weight of said lactam solution.
 15. The process of claim13 in which said water content is from about 2 percent to about 10percent by weight.
 16. The process of claim 5 in which the sulfurdioxide content of the untreated gas stream is from about 0.1 percent toabout 8 percent by volume, the sulfur dioxide impurities in thedischarged gas stream comprising less than about 100 ppm, saidcontacting zone being at least about 10 ft. in length.
 17. The processof claim 16 in which said N-alkyl lactam comprises N-methyl pyrrolidone.18. The process of claim 1 and including removing the N-alkyl lactamhaving sulfur dioxide absorbed therein from the contacting zone andheating to from about 100*C to about 200*C to strip saidsulfur-dioxide-containing gases therefrom and thereafter recycling saidN-alkyl lactam to said contacting zone for contact with additionalquantities of said sulfur dioxide contaminated gas stream, the strippedsulfur dioxide gases being utilized in the production ofsulfur-containing compounds for subsequent use or non-pollutingdisposal.
 19. The process of claim 18 in which said lactam is an N-loweralkyl pyrrolidone.
 20. The process of claim 19 in which said lactamcomprises N-methyl pyrrolidone.
 21. The process of claim 20 in whichsaid gas temperature is from about 35*C to about 65*C.
 22. The processof claim 21 in which said sulfur dioxide-contaminated gas stream is atan elevated temperature above about 65*C and including cooling said gasstream to from about 35*C to about 65*C prior to contact thereof withsaid liquid lactam, said gas mass flow velocity being from about 1,000to about 2,000 lbs/hr/ft2.
 23. The process of claim 22 in which thesulfur dioxide concentration in the treated gas stream discharged to theatmosphere is from about 50 ppm to about 100 ppm, said contacting zonebeing at least about 20 ft. in length, said lactam comprising an aqueoussolution thereof containing up to about 25 percent water by weight ofsolution.
 24. The process of claim 23 in which said lactam comprises anaqueous solution thereof containing from about 2 percent to about 10percent water by weight of solution.
 25. A process for treating a gasstream containing sulfur dioxide impurities comprising: a. contacting anincoming gas stream containing from about 0.01 percent to about 50percent sulfur dioxide by volume with a liquid N-alkyl lactam solvent ina contacting zone at a gas temperature of from about 0*C to about 75*Cat a pressure assuring the flow of said gas stream through thecontacting zone at a gas mass flow velocity of from about 200 to about3, 000 lbs/hr/ft2, the contacting zone having a length of from about 5ft. to about 85 ft.; and b. discharging the thus-treated gas stream fromthe contacting zone, said stream having a sulfur dioxide content of notmore than about 10 percent by volume of the sulfur dioxide content ofthe untreated gas stream, at least about 90 percent by volume of saidsulfur dioxide impurities of the untreated gas stream being removedtherefrom by absorption in said N-alkyl lactam, whereby the sulfurdioxide impurities are very substantially washed from The gas streamcontaining same, the sulfur dioxide impurities being convenientlyavailable for non-polluting disposal or use.
 26. The process of claim 25in which at least about 95 percent of the sulfur dioxide impurities areremoved from the incoming gas stream by absorption in said N-alkyllactam, said gas temperature being from about 35*C to about 65*C. 27.The process of claim 26 in which at least about 98 percent of the sulfurdioxide impurities are removed from the incoming gas stream byabsorption in said N-alkyl lactam.
 28. The process of claim 25 in whichsaid sulfur dioxide content of the untreated gas stream is from about0.1 percent to about 0.5 percent by volume, at least about 95 percent ofthe sulfur dioxide impurities being removed from the incoming gas streamby absorption in said N-alkyl lactam, said gas temperature being fromabout 35*C to about 65*C.
 29. The process of claim 28 in which saidlactam comprises N-methyl pyrrolidone containing up to about 25 percentwater by weight.
 30. The process of claim 29 in which said aqueoussolution contains from about 2 percent to about 10 percent water byweight.
 31. The process of claim 25 in which said sulfurdioxide-containing gas stream is at a temperature above about 65*C andincluding cooling said gas stream to from about 35*C to about 65*C priorto contact thereof with said liquid lactam and including heating lactamremoved from the contacting zone to from about 100*C to about 200*C tostrip said sulfur dioxide-containing gases therefrom and thereafterrecycling said N-alkyl lactam to said contacting zone for contact withadditional quantities of said sulfur dioxide contaminated gas stream,the stripped sulfur dioxide gases being thereafter compressed andrecovered for non-polluting disposal or use.
 32. The process of claim 31in which said contacting zone has a length of at least about 20 ft. 33.The process of claim 32 in which said lactam comprises N-cyclohexylpyrrolidone.
 34. The process of claim 32 in which said lactam comprisesN-methyl pyrrolidone, said lactam removing at least about 95 percent byvolume of the sulfur dioxide impurities contained in the incoming gasstream.
 35. The process of claim 34 in which said lactam removes atleast about 98 percent by volume of the sulfur dioxide impuritiescontained in the incoming gas stream, said absorption zone being atleast about 40 ft. in length.
 36. The process of claim 35 in which saidgas mass flow velocity is from about 1,000 to about 2,000 lbs/hr/ft2.